Unfortunately, I'm in your exact situation, only slightly worse.
I just bought a used Sapphire Nitro+ RX 580 8GB (SN: A171500015272).
When I first ran booted it, it ran flawlessly from Displayport to my monitor. Didn't know much about the card, so didn't notice if the switch was in the left or right BIOS position.
I managed to get windows installed but as soon as it booted into windows the first time, it started to show artifacts on screen. Almost to the point where I wasn't able to read anything on the screen. I tried to download and run GPU-Z, but after a few seconds - black screen. Before this I made it to see a few things i.e. the name which was Ellesmere or something generic like Microsoft Graphics Controller, that made me think, Windows was not even able to identify the card correctly. From that point on both Displayports connectors were now giving no output and had to change to HDMI port.
After that and a windows startup repair, I booted into windows again and Tried to install newest AMD driver pack, but ultimately, that was where it went to shit. Black screen.
I turned off the system and tried to pull the switch back to the left, but it broke off the board. Ouch.
I tried to disassemble the fan sections leaving the heatsink on, but could not find the switch anywhere (neither in my case).
Now I'm unable to turn on the machine and boot - even into BIOS - without significant artifacts on the screen. After the new drivers were installed and the switch broke off, it also rendered my system unable to boot into Windows. Cannot boot into safe mode or reach any workable screen of windows to be able to run DDU.
It just gets into auto-repair mode, thinks for 10 seconds and then black screen (with artifacts all over the screen). So i wiped my M.2 drive with Secure SSD wipe (BIOS function).
All of this is so discouraging. I suspected it's a really faulty VBIOS running right now with completely off settings, (possibly even mining settings?) but can't boot into windows and flash it from there. The fans are running fine at every startup, so no issues there. (can't figure out if they correspond to silent or performance profile).
But what to do now?
The weird thing about this is when I go to BIOS under Graphic Output Protocal (connected via HDMI), it states this:
AMD Radeon Ellesmere Graphics
AMD GOP x64 Release Driver Rev. 1.61.0.15.50. Jan (???)
My current plan:
1) reattempt to install Windows 10 on main NvMe SSD and attempt to flash this stock BIOS:
https://www.techpowerup.com/vgabios/191898/sapphire-rx580-8192-170323
through GPU-Z or AMD VBFLASH utility
If that doesn't work, my alternative plan is:
2) Install Unbuntu Live on second storage and flash vbios the above vbios rom by downloading AMD VBFLASH utility.
3) Can DDU be run from MS-DOS booted from a USB? Perhaps that will solve the artifacts issue and perhaps I should do this first?
I really hope one of you can provide me with a solution or guide me in the right direction.
My system:
Ryzen 5 3600 (No OC, though I tried the Game Boost mode).
MSI B550 Mortar Wifi
2X8GB Corsair Vengeance 3200 MHz CL-16 in dual channel mode
M.2 slot: Samsung PM9A1 SSD PCIe 4.0 NVMe - 512 GB
2. storage: PNY CS900 120 GB
BeQuiet! System Power B9 600W
Monitor: Acer ED273A 144 Hz monitor (VSYNC capable)